High voltage chuck for a probe station

ABSTRACT

A chuck for testing an integrated circuit includes an upper conductive layer having a lower surface and an upper surface suitable to support a device under test. An upper insulating layer has an upper surface at least in partial face-to-face contact with the lower surface of the upper conductive layer, and a lower surface. A middle conductive layer has an upper surface at least in partial face-to-face contact with the lower surface of the upper insulating layer, and a lower surface.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to and is a continuation of U.S. patentapplication Ser. No. 15/072,170, which was filed on Mar. 16, 2016, andissued on Aug. 22, 2017 as U.S. Pat. No. 9,741,599, and which is acontinuation-in-part of U.S. patent application Ser. No. 13/702,054,which was filed on Dec. 4, 2012, and issued on Nov. 29, 2016 as U.S.Pat. No. 9,506,973, and which is a national stage filing under Section371 of International Application No. PCT/US2011/031981, which was filedon Apr. 11, 2011, and which claims the benefit of U.S. ProvisionalApplication Ser. No. 61/352,061 filed on Jun. 7, 2010, and U.S.Provisional Application Ser. No. 61/377,423 filed on Aug. 26, 2010. Thecomplete disclosures of the above-identified patent applications arehereby incorporated by reference.

BACKGROUND OF THE INVENTION

The present invention relates to a chuck suitable for high voltagetesting for a semiconductor wafer, and, more particularly, to waferchuck designs having improved performance over a range of operatingtemperatures and testing conditions.

Processing semiconductor wafers include processes that form a largenumber of devices within and on the surface of the semiconductor wafer(hereinafter referred to simply as “wafer”). After fabrication, thesedevices are typically subjected to various electrical tests andcharacterizations. In some cases, the electrical tests characterize theoperation of circuitry and in other cases characterize the semiconductorprocess. By characterizing the circuitry and devices thereon, the yieldof the semiconductor process may be increased.

Wafer chucks used for high voltage testing may be required to operateacross a wide range of temperature while exhibiting sufficientperformance characteristics such as, for example, thermal uniformityacross the surface of the chuck in contact with the wafer or deviceunder test (OUT), suitable thermal transition time for the particulartests performed, adequate flatness of the chuck surface over the rangeof temperatures used, and low AC and DC electrical noise. Typically,wafer chucks are designed to hold the wafer or device under test using avacuum, and such chucks require substantial supporting structure andassociated equipment for accurately positioning the device under test ina controlled manner and for doing so within a controlled environment.

Changes in chuck design to accomplish a particular requirement may haveadverse effects on costs, quality and/or testing processing times. Forexample, adding material to the chuck may increase costs for theparticular material added, require additional thermal controls (such asadditional chiller equipment for the probe station), add testingprocessing time due to an increase in thermal mass and decrease thermaltransition time, contribute non-uniformities in thermal characteristicsto the devices under test (thus decreasing testing quality andaccuracy), and add to the overall physical space requirements within theprobe station (causing other components to require resizing or increasedsizes).

What is needed, therefore, are improved chuck designs that address theseand other challenges. The foregoing and other objectives, features andadvantages of the invention will be more readily understood uponconsideration of the following detailed description of the inventiontaken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

For a more complete understanding of the present invention, the drawingsherein illustrate examples of the invention. The drawings, however, donot limit the scope of the invention. Similar references in the drawingsindicate similar elements.

FIG. 1 is a partial front view of an exemplary embodiment of a waferprobe station constructed in accordance with the present invention.

FIG. 2A is a partial top view of the wafer probe station of FIG. 1 withthe enclosure door shown partially open.

FIG. 2B is a top view of the wafer probe station of FIG. 1.

FIG. 3A is an enlarged sectional view taken along line 3A-3A of FIG. 3B.

FIG. 3B is a partially sectional and partially schematic front view ofthe probe station of FIG. 1.

FIG. 4 is a top view of the sealing assembly where the motorizedpositioning mechanism extends through the bottom of the enclosure.

FIG. 5A is an enlarged top detail view taken along line 5A-5A of FIG. 1.

FIG. 5B is an enlarged top sectional view taken along line 5B-5B of FIG.1.

FIG. 6 is a partially schematic top detail view of the chuck assembly,taken along line 6-6 of FIG. 3.

FIG. 6a illustrates a vacuum hole pattern for a planar upwardly-facingwafer-supporting surface of an upper chuck assembly element.

FIG. 6b illustrates a bottom surface vacuum channel pattern for thedownwardly-facing surface of the upper chuck assembly element shown inFIG. 6 a.

FIG. 6c illustrates another vacuum hole pattern for a planarupwardly-facing wafer-supporting surface of an upper chuck assemblyelement.

FIG. 6d illustrates a bottom surface vacuum channel pattern for thedownwardly-facing surface of the upper chuck assembly element shown inFIG. 6 c.

FIG. 6e is a top perspective view of a porous planar upwardly-facingwafer-supporting surface of an upper chuck assembly element.

FIG. 6f is a top perspective view of a vacuum distributing plate elementfor use with a porous wafer-supporting surface as in FIG. 6 e.

FIG. 6g is a top view of a grooved planar upwardly-facingwafer-supporting surface of an upper chuck assembly element.

FIG. 7 is a partially sectional front view of the chuck assembly of FIG.6.

FIG. 8 illustrates a high voltage chuck.

FIG. 9 illustrates a breakdown voltage versus pressure x gap curve.

FIG. 10 illustrates an upper portion of the high voltage chuck withvacuum lines.

FIG. 11 illustrates a middle portion of the high voltage chuck withvacuum lines.

FIG. 12 illustrates a lower portion of the high voltage chuck withvacuum lines.

FIG. 13 illustrates a high voltage chuck with vacuum lines.

FIG. 14 illustrates a modified chuck.

FIG. 15 illustrates another modified chuck.

FIG. 16 illustrates another modified chuck.

FIG. 17 illustrates a high voltage chuck with insulated and guarded liftpin.

FIG. 18 illustrates a high voltage chuck with insulated lift pin.

FIG. 19A illustrates an exemplary thermal chuck with auxiliary chucks,guard ring, and other structures in close proximity to the chuck topforce/sense surface.

FIG. 19B illustrates an exemplary prober chamber with a guard ring andother chamber structures in close proximity to the chuck top force/sensesurface.

FIG. 19C illustrates an exemplary prober chamber with an upper guard, aguard ring, and other chamber structures in close proximity to the chucktop force/sense surface.

FIG. 20A illustrates a chuck top force/sense connection.

FIG. 20B is a transparent view of the connection shown in FIG. 20A.

FIG. 20C is a side view of the connection shown in FIG. 20A.

FIG. 20D is a sectional view of the connection shown in FIG. 20A.

FIG. 20E is a cut view of the connection shown in FIG. 20A.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

In the following detailed description, numerous specific details are setforth in order to provide a thorough understanding of the preferredembodiments. However, those skilled in the art will understand that thepresent invention may be practiced without these specific details, thatthe present invention is not limited to the depicted embodiments, andthat the present invention may be practiced in a variety of alternateembodiments. In other instances, well-known methods, procedures,components and systems have not been described in detail.

In many cases a probe station, such as those available from CascadeMicrotech, Inc., are used to perform the characterization of thesemiconductor process. With reference to FIGS. 1, 2 and 3, a probestation comprises a base 10 (shown partially) which supports a platen 12through a number of jacks 14 a, 14 b, 14 c, 14 d which selectively raiseand lower the platen vertically relative to the base by a smallincrement (approximately one tenth of an inch) for purposes to bedescribed hereafter. Also supported by the base 10 of the probe stationis a motorized positioner 16 having a rectangular plunger 18 whichsupports a movable chuck assembly 20 for supporting a wafer or othertest device. The chuck assembly 20 passes freely through a largeaperture 22 in the platen 12 which permits the chuck assembly to bemoved independently of the platen by the positioner 16 along X, Y and Zaxes, i.e., horizontally along two mutually perpendicular axes X and Y,and vertically along the Z axis. Likewise, the platen 12, when movedvertically by the jacks 14, moves independently of the chuck assembly 20and the positioner 16.

Mounted atop the platen 12 are multiple individual probe positionerssuch as 24 (only one of which is shown), each having an extending member26 to which is mounted a probe holder 28 which in turn supports arespective probe 30 for contacting wafers and other test devices mountedatop the chuck assembly 20. The probe positioner 24 has micrometeradjustments 34, 36 and 38 for adjusting the position of the probe holder28, and thus the probe 30, along the X, Y and Z axes, respectively,relative to the chuck assembly 20. The Z axis is exemplary of what isreferred to herein loosely as the “axis of approach” between the probeholder 28 and the chuck assembly 20, although directions of approachwhich are neither vertical nor linear, along which the probe tip andwafer or other test device are brought into contact with each other, arealso intended to be included within the meaning of the term “axis ofapproach.” A further micrometer adjustment 40 adjustably tilts the probeholder 28 to adjust planarity of the probe with respect to the wafer orother test device supported by the chuck assembly 20. As many as twelveindividual probe positioners 24, each supporting a respective probe, maybe arranged on the platen 12 around the chuck assembly 20 so as toconverge radially toward the chuck assembly similarly to the spokes of awheel. With such an arrangement, each individual positioner 24 canindependently adjust its respective probe in the X, Y and Z directions,while the jacks 14 can be actuated to raise or lower the platen 12 andthus all of the positioners 24 and their respective probes in unison.

An environment control enclosure (fully sealed, partially sealed, orotherwise) is composed of an upper box portion 42 rigidly attached tothe platen 12, and a lower box portion 44 rigidly attached to the base10. Both portions are made of steel or other suitable electricallyconductive material to provide EMI shielding. To accommodate the smallvertical movement between the two box portions 42 and 44 when the jacks14 are actuated to raise or lower the platen 12, an electricallyconductive resilient foam gasket 46, preferably composed of silver orcarbon impregnated silicone, is interposed peripherally at their matingjuncture at the front of the enclosure and between the lower portion 44and the platen 12 so that an EMI, substantially hermetic, and light sealare all maintained despite relative vertical movement between the twobox portions 42 and 44. Even though the upper box portion 42 is rigidlyattached to the platen 12, a similar gasket 47 is preferably interposedbetween the portion 42 and the top of the platen to maximize sealing.

With reference to FIGS. 5A and 5B, the top of the upper box portion 42comprises an octagonal steel box 48 having eight side panels such as 49a and 49 b through which the extending members 26 of the respectiveprobe positioners 24 can penetrate movably. Each panel comprises ahollow housing in which a respective sheet 50 of resilient foam, whichmay be similar to the above identified gasket material, is placed. Slitssuch as 52 are partially cut vertically in the foam in alignment withslots 54 formed in the inner and outer surfaces of each panel housing,through which a respective extending member 26 of a respective probepositioner 24 can pass movably. The slitted foam permits X, Y and Zmovement of the extending members 26 of each probe positioner, whilemaintaining the EMI, substantially hermetic, and light seal provided bythe enclosure. In four of the panels, to enable a greater range of X andY movement, the foam sheet 50 is sandwiched between a pair of steelplates 55 having slots 54 therein, such plates being slidabletransversely within the panel housing through a range of movementencompassed by larger slots 56 in the inner and outer surfaces of thepanel housing.

Atop the octagonal box 48, a circular viewing aperture 58 is provided,having a recessed circular transparent sealing window 60 therein. Abracket 62 holds an apertured sliding shutter 64 to selectively permitor prevent the passage of light through the window. A stereoscope (notshown) connected to a CRT monitor can be placed above the window toprovide a magnified display of the wafer or other test device and theprobe tip for proper probe placement during set-up or operation.Alternatively, the window 60 can be removed and a microscope lens (notshown) surrounded by a foam gasket can be inserted through the viewingaperture 58 with the foam providing EMI, hermetic and light sealing. Theupper box portion 42 of the environment control enclosure also includesa hinged steel door 68 which pivots outwardly about the pivot axis of ahinge 70 as shown in FIG. 2A. The hinge biases the door downwardlytoward the top of the upper box portion 42 so that it forms a tight,overlapping, sliding peripheral seal 68 a with the top of the upper boxportion. When the door is open, and the chuck assembly 20 is moved bythe positioner 16 beneath the door opening as shown in FIG. 2A, thechuck assembly is accessible for loading and unloading.

With reference to FIGS. 3 and 4, the sealing integrity of the enclosureis likewise maintained throughout positioning movements by the motorizedpositioner 16 due to the provision of a series of four sealing plates72, 74, 76 and 78 stacked slidably atop one another. The sizes of theplates progress increasingly from the top to the bottom one, as do therespective sizes of the central apertures 72 a, 74 a, 76 a and 78 aformed in the respective plates 72, 74, 76 and 78, and the aperture 79 aformed in the bottom 44 a of the lower box portion 44. The centralaperture 72 a in the top plate 72 mates closely around the bearinghousing 18 a of the vertically movable plunger 18. The next plate in thedownward progression, plate 74, has an upwardly projecting peripheralmargin 74 b which limits the extent to which the plate 72 can slideacross the top of the plate 74. The central aperture 74 a in the plate74 is of a size to permit the positioner 16 to move the plunger 18 andits bearing housing 18 a transversely along the X and Y axes until theedge of the top plate 72 abuts against the margin 74 b of the plate 74.The size of the aperture 74 a is, however, too small to be uncovered bythe top plate 72 when such abutment occurs, and therefore a seal ismaintained between the plates 72 and 74 regardless of the movement ofthe plunger 18 and its bearing housing along the X and Y axes. Furthermovement of the plunger 18 and bearing housing in the direction ofabutment of the plate 72 with the margin 74 b results in the sliding ofthe plate 74 toward the peripheral margin 76 b of the next underlyingplate 76. Again, the central aperture 76 a in the plate 76 is largeenough to permit abutment of the plate 74 with the margin 76 b, butsmall enough to prevent the plate 74 from uncovering the aperture 76 a,thereby likewise maintaining the seal between the plates 74 and 76.Still further movement of the plunger 18 and bearing housing in the samedirection causes similar sliding of the plates 76 and 78 relative totheir underlying plates into abutment with the margin 78 b and the sideof the box portion 44, respectively, without the apertures 78 a and 79 abecoming uncovered. This combination of sliding plates and centralapertures of progressively increasing size permits a full range ofmovement of the plunger 18 along the X and Y axes by the positioner 16,while maintaining the enclosure in a sealed condition despite suchpositioning movement. The EMI sealing provided by this structure iseffective even with respect to the electric motors of the positioner 16,since they are located below the sliding plates.

With particular reference to FIGS. 3, 6 and 7, the chuck assembly 20 isa modular construction usable either with or without an environmentcontrol enclosure. The plunger 18 supports an adjustment plate 79 whichin turn supports first, second and third chuck assembly elements 80, 81and 83, respectively, positioned at progressively greater distances fromthe probe(s) along the axis of approach. Element 83 is a conductiverectangular stage or shield 83 which detachably mounts conductiveelements 80 and 81 of circular shape. The element 80 has a planarupwardly facing wafer supporting surface 82 having an array of verticalapertures 84 therein. These apertures communicate with respectivechambers separated by O-rings 88, the chambers in turn being connectedseparately to different vacuum lines 90 a, 90 b, 90 c (FIG. 6)communicating through separately controlled vacuum valves (not shown)with a source of vacuum. The respective vacuum lines selectively connectthe respective chambers and their apertures to the source of vacuum tohold the wafer, or alternatively isolate the apertures from the sourceof vacuum to release the wafer, in a conventional manner. The separateoperability of the respective chambers and their corresponding aperturesenables the chuck to hold wafers of different diameters.

In addition to the circular elements 80 and 81, auxiliary chucks such as92 and 94 are detachably mounted on the corners of the element 83 byscrews (not shown) independently of the elements 80 and 81 for thepurpose of supporting contact substrates and calibration substrateswhile a wafer or other test device is simultaneously supported by theelement 80. Each auxiliary chuck 92, 94 has its own separate upwardlyfacing planar surface 100, 102 respectively, in parallel relationship tothe surface 82 of the element 80. Vacuum apertures 104 protrude throughthe surfaces 100 and 102 from communication with respective chamberswithin the body of each auxiliary chuck. Each of these chambers in turncommunicates through a separate vacuum line and a separate independentlyactuated vacuum valve (not shown) with a source of vacuum, each suchvalve selectively connecting or isolating the respective sets ofapertures 104 with respect to the source of vacuum independently of theoperation of the apertures 84 of the element 80, so as to selectivelyhold or release a contact substrate or calibration substrate located onthe respective surfaces 100 and 102 independently of the wafer or othertest device. An optional metal shield 106 may protrude upwardly from theedges of the element 83 to surround the other elements 80, 81 and theauxiliary chucks 92, 94.

The apertures 84 of the element 80 may be positioned as shown in FIG. 6for the surface 82, for conducting sufficiently strong vacuum to hold awafer (not shown). More preferably, a greater number of holes(apertures) 84 a are used, positioned on the surface 82 a as (partially)shown in FIG. 6a for improved holding of a wafer. The bottom surface 82b of the element 80 may be as shown in FIG. 6 b, having machinedchannels or grooves typically, in various designs, of widths varyingfrom 300 to 3000 microns to conduct the vacuum. For example, a vacuumline may conduct a vacuum from an upper chuck element edge port 600 bwhich is cross-drilled into the edge of the chuck element and connectedwith machined groove 610 b, shown extending in a circular manner aroundthe bottom surface 82 b and interconnecting with through holes(apertures) 84 b extending to the upper (device under test engaging)chuck surface 82 a. Several similar machined grooves are shown extendingfrom near the edge of the bottom surface 82 b toward the center of thebottom surface 82 b, and interconnected with circular machined groovesand multiple through holes extending to the upper surface 82 a. Themachined groove 620 b, for example, extends from near the edge to thecenter of the (circular) bottom surface 82 b and interconnects severalholes 84 b near the center of the surface.

The vacuum hole positions and corresponding machined grooves shown inFIGS. 6a and 6 b, respectively, may be sufficient for holding thickwafers that bridge over multiple holes 84 a, but may not be sufficientfor pulling down and holding warped wafers, thinned wafers, or shards ofwafers. Preferably, wafers and devices under test should be held to thechuck upper surface (such as surface 82 a) with high force foruniformity of electrical contact with the wafer or device under test.The area in contact on the device under test is a variable in themeasured resistance and is to be held as constant as possible. Machinedgrooves directly under the wafer or device under test (to the extentthat the upper chuck surface comprises machined grooves instead of or inaddition to vacuum holes for conducting vacuum to pull down the wafer ordevice under test) comprise voids under the wafer that are large enoughto create errors in electrical measurements due to changes in contactresistance and in the creation of RF noise. Likewise, vacuum holes maycomprise similar voids (sometimes referred to as RF voids), andtherefore similar sources of error. Further, if the contact force of thewafer to the chuck varies due to non-uniformities in vacuum across thewafer, measurements of the device under test will vary. Such measurementvariations can invalidate modeling of the device under test.

A vacuum hole for holding a wafer to the chuck has a limited area ofeffect where the full force of the vacuum is applied. Preferably, thevacuum holes positioned across the wafer-engaging surface of the upperchuck element are spaced close enough to have overlapping areas of fulleffect and uniform spacing around the entire wafer-engaging surface.Preferably, to achieve a full and uniform vacuum effect the vacuum holesare spaced approximately 0.38 inches apart or less over the entire chuckwafer/OUT-engaging surface. Also preferably, the vacuum may be appliedto multiple zones in the chuck top allowing shards and different sizedwafers to be held with sufficient vacuum.

In preferred embodiments, the wafer-engaging surface (or chuck top) 82c, as shown in FIG. 6 c, includes apertures 84 c that are positionedwith relation to one another so as to (collectively) create overlappingareas of vacuum effect across the wafer-engaging portion (or the entirewafer) of the surface 82 c. The chuck top 82 c effectively provides whatmay be described as an “uninterrupted hole pattern chuck top” forholding wafers during testing. Such a hole pattern allows for deliveryof a stronger and more uniform level of vacuum pressure to the supportedwafer (and devices under test thereon). The stronger vacuum pressure andmore uniform distribution across the wafer surface improve theelectrical contacts and characteristics associated with probing andtesting the wafer and the devices under test thereon. For example, achuck top surface 82 c as in FIG. 6c allows for more uniform/lowerelectrical contact resistance and inductance, more uniform contactareas, more even vacuum distribution across each device under test andbetween each device under test, more complete or full mechanical supportfor thinned wafers or shards of wafers, and for use of smaller aperturesizes and thus smaller voids presented to the underside of the wafer,thereby further reducing sources of RF noise.

FIG. 6d depicts one embodiment of a bottom surface vacuum channelpattern for the downwardly facing surface of the upper chuck assemblyelement shown in FIG. 6 c. As shown, the bottom surface 82 dincorporates machined (or otherwise formed) channels which conductvacuum from holes 84 d that penetrate through to corresponding apertures84 c as shown in FIG. 6 c. The holes 84 d are positioned so as to bespaced apart in a substantially correspondingly opposite fashion as forthe surface 82 c, with circular machined grooves (or channels)interconnecting regions of holes 84 d. In preferred embodiments, deeplycross-drilled inner channels conduct vacuum to the circular grooves, andselectively and controllably applying vacuum to the inner channelsallows for maintaining strong and uniformly applied vacuum across thewafer-engaging surface 82 c. Such deep cross-drilling of a chuck topplate as shown in FIGS. 6c and 6d has heretofore been viewed as toodifficult (to controllably and accurately cross-drill as much as fourinches or more into a 0.23 inch thick chuck top) or unnecessary becausethe characteristics associated with stronger and more uniform vacuum,flatness of the chuck top surface, impact of top surface voids, andother characterizations were not previously discovered.

A vacuum line, for example, may conduct a vacuum from an upper chuckelement edge port 600 d, as indicated in FIG. 6 d, which iscross-drilled into the edge of the chuck element (or otherwise formed asan inner channel) and connected with machined (or etched or otherwiseformed) groove 610 d, shown extending in a circular manner around thebottom surface 82 d and interconnecting with through holes (apertures)84 d that are proximate to the groove 610 d and extend to the upper(device under test engaging) chuck surface 82 c. Preferably, multipleinner channels are used to discretely and separately control vacuum forseparate groups of holes 84 d. For example, vacuum from one group ofholes such as those interconnected with the outermost circular channelmay be selectively controlled by edge vacuum connection 600 d; vacuumfrom another group of holes such as those connected with one or more ofthe interior surface circular channels may be selectively controlled byedge vacuum connection 602 d; and yet another group of holes such asthose centrally positioned holes 620 d may be selectively controlled byedge vacuum connection 604 d. Each of the different cross-drilledinternal vacuum lines may be connected at the edge connections 600 d,602 d, 604 d with vacuum lines such as the lines 90 a, 90 b, 90 cdescribed and shown in FIG. 6. Auxiliary chucks such as aux chucks 92and 94 also shown in FIG. 6, or any wafer or device under test engagingchuck surface having vacuum holes may incorporate the vacuum and otherrelated improvements described herein. For example, the aux chuck 92, 94holes 104 preferably comprise holes positions as in FIG. 6c for improvedperformance.

In preferred embodiments, a porous sintered metal chuck top 82 e asshown in FIG. 6e may be used as a vacuum distributor. A wafer-engagingsurface such as the chuck top surface 82 e preferably incorporates acontinuous and uniform layer 630 e of porous (conductive) materialhaving micron sized pores through which a vacuum may be drawn so as tostrongly hold a wafer or device under test thereon. Supporting theporous layer 630 e is preferably a vacuum distributing (grooved) plate640 e having a surface 82 f as shown in FIG. 6 f. The grooved plate 640e preferably includes (machined, etched, or otherwise formed) surfacechannels for conducting a vacuum for the porous layer 630 e. Similar tothe channels and cross-drilled internal vacuum lines shown and describedin FIG. 6 d, the grooved plate 640 e preferably includes circularchannels 610 f forming groups of channels which are interconnected withinternal cross-drilled vacuum lines (not shown). Vacuum lines may beconnected to the edge connections (not shown) for each of the separatelycontrollable vacuum lines and channels so as to allow selective controlof the vacuum provided to particular regions of pores/holes 84 e (orvacuum zones) in the porous layer 630 e.

Alternatively, instead of cross-drilled internal vacuum lines, throughholes providing vacuum to each of the separate regions may be used. Forexample, the vacuum channel 610 f and other channels on the groovedplate 640 e may be supplied vacuum using through holes extending fromthe channel 610 f on the surface 82 f to the back or lower side of thegrooved plate 640 e. The through holes (not shown) may then beinterconnected with channels formed in the back or lower side of thegrooved plate 640 e or with vacuum paths from lower layers (or stages)of the chuck.

A porous wafer-engaging surface such as porous surface 82 e allows forgreater uniformity of vacuum force applied to the wafer or device undertest, and flexibility for holding shards of wafers, warped wafers, orwafers of varying sizes. Further, the porous surface 82 e improves theelectrical contact characteristics for the device under test. The microsized pores and uniform vacuum improves support of the wafer in theareas of the device under test that may include measurement pads andtest pad structures. If the wafer surface under the device under test isnot fully supported due to voids or leakage of vacuum due to roughnessof the lower surface of the wafer, there may be, as previouslymentioned, variation in the contact force of the wafer to the (porous)chuck surface and, consequently, variations in the measurements whichmay invalidate modeling of the device under test. The micron sizedpores/holes in the porous surface 82 e create voids under the wafer anddevice under test which are less significant relative to the pitch ofthe measurement pads and test pad structures typically used.

A grooved (or micro-grooved) planar upwardly-facing wafer-supportingsurface 82 g of an upper chuck assembly element is shown in FIG. 6 g.The micro-grooved surface 82 g incorporates vacuum grooves 84 g,preferably at least as small as 50 microns wide by 15 microns deep. Thegrooves may be machined, chemically etched, laser cut, or otherwiseformed within the wafer-engaging surface 82 g, and the grooves arepreferably sized so as to be nearly as narrow (i.e. 50 microns or less)as the thinnest of wafers typically used in semiconductor processing.The grooves 84 g may be supplied vacuum as described and shown in FIGS.6a -6 f. Preferably, the pattern of grooves 84 g is substantially asshown in FIG. 6 g, comprising circular grooves that are closely spacedwith respect to one another and interconnected in groups (or zones) ofgrooves that are separately selectively controllable so as to allowsufficient vacuum retention of irregular-sized shards and differentsized wafers.

Generally, wafers are preferably held to the chuck surface 82 g withhigh force for uniformity of electrical contact. The area in contact onthe device under test (OUT) is a variable in the OUT's measuredresistance and is preferably held constant. Conventionally machinedgrooves are typically 300 to 3000 microns wide and comprise cavities (orvoids) under the wafer that are large enough to create errors inelectrical measurements due to changes in contact resistance and in thecreation of RF noise. The cavities created under the wafer with suchtypically machined vacuum grooves may cause variations in thetermination of the measurements electric field and of the fringingcapacitance of the measurement. These variations may invalidate modelingof the device under test.

The grooves in the micro-grooved surface 82 g, however, are preferablyat least as small as 50 microns (in width) which is nearly as narrow asthe thinnest wafers typically used in semiconductor processing. Themicro-grooves 84 g are preferably positioned with close enough spacingso as to provide a continuous or uniform vacuum field and uniformcontact area for supporting the wafer with no voids/cavities which maybe significant in size relative to the pitch of measurement pads used.Preferably, the percentage of the area of the wafer directly below thedevice under test and test pad structures that are not in contact withwafer-supporting portions of the surface 82 g (such as the area over avacuum groove) is kept very small (i.e. insignificant) as compared tothe percentage of required error in the measurement.

Conventional wafer vacuum chucks typically use machined grooves varyingfrom 300 to 3000 microns in width to conduct the vacuum directly underthe wafer or use an added top plate with a limited number of discreteholes/apertures to conduct the vacuum. The large and widely spacedmachined grooves are better suited for holding thick wafers that canbridge over the groove and may exert sufficiently strong vacuum for suchthick wafers. However, such designs may fail to pull down and adequatelyhold warped wafers or shards of wafers and may cause physical damage tothin wafers if the force of vacuum is too great. Using vacuum holesinstead of machined grooves in the planar upwardly-facingwafer-supporting surface of an upper chuck assembly may reduce themagnetic/electric field void and thin wafer damage concerns.

The micro-grooved surface 82 g as shown in FIG. 6g preferably combinesthe strong vacuum holding capability of grooved wafer-engaging surfacedesigns and the thin wafer mechanical/physical support and electric andmagnetic field advantages of vacuum hole wafer-engaging surface designs,while improving uniformity of vacuum holding force and uniformity ofmechanical and electrical contact characteristics with the wafer.

All of the chuck assembly elements 80, 81 and 83, as well as theadditional chuck assembly element 79, are electrically insulated fromone another even though they are constructed of electrically conductivemetal and interconnected detachably by metallic screws such as 96. Withreference to FIGS. 3A and 3B, the electrical insulation results from thefact that, in addition to the resilient dielectric O-rings 88,dielectric spacers 85 and dielectric washers 86 are provided. These,coupled with the fact that the screws 96 pass through oversizedapertures in the lower one of the two elements which each screw joinstogether thereby preventing electrical contact between the shank of thescrew and the lower element, provide the desired insulation. As isapparent in FIG. 3A, the dielectric spacers 85 extend over only minorportions of the opposing surface areas of the interconnected chuckassembly elements, thereby leaving air gaps between the opposingsurfaces over major portions of their respective areas. Such air gapsminimize the dielectric constant in the spaces between the respectivechuck assembly elements, thereby correspondingly minimizing thecapacitance between them and the ability for electrical current to leakfrom one element to another. Preferably, the spacers and washers 85 and86, respectively, are constructed of a material having the lowestpossible dielectric constant consistent with high dimensional stabilityand high volume resistivity. A suitable material for the spacers andwashers is glass epoxy, or acetyl homopolymer marketed under thetrademark Delrin by E. I. DuPont.

With reference to FIGS. 6 and 7, the chuck assembly 20 also includes apair of detachable electrical connector assemblies designated generallyas 108 and 110, each having at least two conductive connector elements108 a, 108 b and 110 a, 110 b, respectively, electrically insulated fromeach other, with the connector elements 108 b and 110 b preferablycoaxially surrounding the connector elements 108 a and 110 a as guardstherefore. If desired, the connector assemblies 108 and 110 can betriaxial in configuration so as to include respective outer shields 108c, 110 c surrounding the respective connector elements 108 b and 110 b,as shown in FIG. 7. The outer shields 108 c and 110 c may, if desired,be connected electrically through a shielding box 112 and a connectorsupporting bracket 113 to the chuck assembly element 83, although suchelectrical connection is optional particularly in view of thesurrounding EMI shielding enclosure 42, 44. In any case, the respectiveconnector elements 108 a and 110 a are electrically connected inparallel to a connector plate 114 matingly and detachably connectedalong a curved contact surface 114 a by screws 114 b and 114 c to thecurved edge of the chuck assembly element 80. Conversely, the connectorelements 108 b and 110 b are connected in parallel to a connector plate116 similarly matingly connected detachably to element 81. The connectorelements pass freely through a rectangular opening 112 a in the box 112,being electrically insulated from the box 112 and therefore from theelement 83, as well as being electrically insulated from each other. Setscrews such as 118 detachably fasten the connector elements to therespective connector plates 114 and 116.

Either coaxial or, as shown, triaxial cables 119 and 120 form portionsof the respective detachable electrical connector assemblies 108 and110, as do their respective triaxial detachable connectors 122 and 124which penetrate a wall of the lower portion 44 of the environmentcontrol enclosure so that the outer shields of the triaxial connectors122, 124 are electrically connected to the enclosure. Further triaxialcables 122 a, 124 a are detachably connectable to the connectors 122 and124 from suitable test equipment such as a Hewlett Packard 41428 modularDC source/monitor or a Hewlett Packard 4284A precision LCR meter,depending upon the test application. If the cables 119 and 120 aremerely coaxial cables or other types of cables having only twoconductors, one conductor interconnects the inner (signal) connectorelement of a respective connector 122 or 124 with a respective connectorelement 108 a or 110 a, while the other conductor connects theintermediate (guard) connector element of a respective connector 122 or124 with a respective connector element 108 b, 110 b.

With sufficiently high probing voltages, such as 5,000 volts, 10,000volts, or more, the different layers of the chuck tend to arc orotherwise short with one another. In addition, with such sufficientlyhigh probing voltages the different layers of the chuck tend to arc orotherwise short with other structures in the vicinity to the chuck. Ineither case, a suitable chuck is required for probing at such extremevoltage levels.

One technique to increase the voltage capabilities of a chuck is tosignificantly increase the thickness of each of the layers within thechuck. While this may appear to be an appropriate technique, however,such a single pronged technique has significant limitations. Thisresults in a significantly thicker chuck assembly that may be too thickto be operational within existing probing stations. In many cases, it isdesirable to test integrated circuits at significantly elevatedtemperature such as 200 degrees Celsius, 300 degrees Celsius, or more.As the mass of the chuck increases, it becomes increasingly moredifficult to accurately control the temperature of the chuck. Inparticular, it becomes increasingly more difficult to maintain the uppersurface of the chuck at a desired temperature. Accordingly, it isgenerally undesirable to substantially increase the thickness of thelayers of the chuck.

In general, as the distance between a pair of conductors separated by anair gap increases, the voltage level that is necessary to cause abreakdown between the spaced apart conductors increases. In this manner,it is generally preferable for a chuck to have conductors or otherconductive members spaced apart by a sufficient distance to reduce thelikelihood of high voltage breakdown. This provides a design criteriafor a high voltage chuck.

In general, creepage is the shortest path between two conductive parts(or between a conductive part and the bounding surface) measured alongthe surface of the insulation between the two conductive parts. With asufficiently high applied voltage, the creepage results in a partiallyconducting path of localized deterioration on the surface of aninsulating material as a result of the electric discharges on or closeto an insulation surface. In this manner, it is generally preferable fora chuck to have conductors or other conductive members spaced apart asufficient distance to reduce the likelihood of high voltage breakdownas a result of creepage. In general, the creepage distance is twice thatof the breakdown as a result of an air gap. This provides yet anotherdesign criteria for a high voltage chuck.

In addition, as the temperature at which testing occurs increases thegreater the distance that is required between conductive members and/orcreepage distances. In this manner, at high testing temperatures thedesign problems are increased.

Referring to FIG. 8, a chuck 500 typically includes a planar upper chuckassembly element 502 the upper surface 504 of which is suitable tosupport a device under test (OUT) 506. The upper chuck assembly element502 and/or device under test 506 is preferably connected to a signalpotential. For a chuck suitable for controlled high temperaturemeasurements, the upper chuck element 502 is preferably constructed fromaluminum and is approximately 0.23 inches thick. In general, the upperchuck element 502 is preferably from approximately 0.1 inches thick toapproximately 0.5 inches thick, in order to maintain a relativelycompact chuck and maintain a relatively low mass chuck assembly wherethe temperature is more readily controllable. Other materials andthicknesses may likewise be used. It is noted that FIG. 8 and othersimilar figures are for illustration and thus not to scale.

The chuck typically also includes a planar middle chuck assembly element508. The middle chuck assembly element is preferably connected to aguard potential. For a chuck suitable for controlled high temperaturemeasurements, the middle chuck assembly element 508 is preferablyrelatively thin, such as 0.01 inch thick conductive foil. In general,the middle chuck assembly element 508 is preferably from approximately0.05 to 0.02 inches thick in order to maintain a relatively compactchuck and maintain a relatively low mass chuck assembly where thetemperature is more readily controllable. Other materials andthicknesses may likewise be used.

The chuck typically also includes an upper chuck insulating element 510positioned between the upper chuck assembly element 502 and the middlechuck assembly element 508. For a chuck suitable for controlled hightemperature measurements the upper chuck insulating element 510 ispreferably constructed from Boron Nitride having a dielectric constantof approximately 4.1. For a chuck suitable for controlled hightemperature measurements, the upper chuck insulating element 510 ispreferably relatively thin, such as 0.17 inches thick. In general, theupper chuck insulating element 510 is preferably from approximately 0.05to 0.4 inches thick in order to maintain a relatively compact chuck andmaintain a relatively low mass chuck assembly where the temperature ismore readily controllable. In addition, the upper chuck insulatingelement 510 preferably has a dielectric constant from approximately 3 toapproximately 6. Other materials and thicknesses may likewise be used.

With such high voltages potentially being applied during testing, it isdesirable to increase the arc distance between the upper chuck assemblyelement 502 and the middle chuck assembly element 508 by extending theupper chuck insulating element 510 past the exterior surface 512 of theupper chuck assembly element 502 and past the exterior surface 514 ofthe middle chuck assembly element 508. The extension 516 of the upperchuck insulating element 510 increases the arc distance and increasesthe creepage distance between the upper and middle chuck assemblyelements 502 and 508, without increasing the thickness of the chuck orotherwise substantially increasing the mass of the chuck assembly. Theextension 516 of the upper chuck insulating element 510 is preferably atleast approximately 0.1 inches, and more preferably approximately 0.25inches, and preferably less than approximately 0.75 inches. Depending onthe thickness of the upper chuck insulating element 510, the extension516 is preferably approximately the same thickness of the upper chuckinsulating element 510, more preferably approximately 2-3 times thethickness of the upper chuck insulating element 510, and more preferablyapproximately no greater than 5 times the thickness of the upper chuckinsulating element 510. Other materials and thicknesses may likewise beused.

The chuck typically also includes a lower chuck insulating element 520positioned between the middle chuck assembly element 508 and a lowerchuck assembly element 522. For a chuck suitable for controlled hightemperature measurements, the lower chuck insulating element 520 ispreferably constructed from Boron Nitride having a dielectric constantof 4.4. For a chuck suitable for controlled high temperaturemeasurements, the lower chuck insulating element 520 is preferablyrelatively thin, such as 0.17 inch thick. In general, the lower chuckinsulating element 520 is preferably from approximately 0.05 to 0.3inches thick in order to maintain a relatively compact chuck andmaintain a relatively low mass chuck assembly where the temperature ismore readily controllable. In addition, the lower chuck insulatingelement 520 preferably has a dielectric constant from approximately 3 toapproximately 6. Other materials and thicknesses may likewise be used.

The chuck typically also includes a planar lower chuck assembly element522. The lower chuck assembly element 522 is preferably connected to ashield and/or ground potential. For a chuck suitable for controlled hightemperature measurements, the lower chuck assembly element 522 ispreferably relatively thin, such as 0.01 inch thick conductive foil. Ingeneral, the lower chuck assembly element 522 is preferably fromapproximately 0.005 to 0.02 inches thick in order to maintain arelatively compact chuck and maintain a relatively low mass chuckassembly where the temperature is more readily controllable. Othermaterials and thicknesses may likewise be used.

With such high voltages potentially being applied during testing, thechuck may increase the arc distance between the middle chuck assemblyelement 508 and the lower chuck assembly element 522 by extending thelower chuck insulating element 520 past the exterior surface 524 of thelower chuck assembly element 522 and past the exterior surface 514 ofthe middle chuck assembly element 508. The extension 526 of the lowerchuck insulating element 520 increases the arc distance and increasesthe creepage distance between the middle and lower chuck assemblyelements 508 and 522, without increasing the thickness of the chuck orotherwise substantially increasing the mass of the chuck assembly. Theextension 526 of the lower chuck insulating element 520 is preferably atleast approximately 0.1 inches, and more preferably approximately 0.25inches, and preferably less than approximately 0.75 inches. Depending onthe thickness of the lower chuck insulating element 520, the extension526 is preferably approximately the same thickness of the lower chuckinsulating element 520, more preferably approximately 2-3 times thethickness of the lower chuck insulating element 520, and more preferablyapproximately no greater than 5 times the thickness of the lower chuckinsulating element 520. Other materials and thicknesses may likewise beused. In many cases, the extension 526 of the lower chuck insulatingelement 520 may be omitted.

The chuck 500 may be supported by a thermal chuck 540. The thermal chuck540 may increase and/or decrease the temperature applied to the chuck500 supported thereon. The thermal chuck may apply any suitabletemperature, such as for example, 200 degrees Celsius, 300 degreesCelsius, or more. In order to maintain the various layers of the chuck500 in their various positions relative to one another, it is desirablethat this is accomplished in a manner that does not otherwisesignificantly degrade the performance of the chuck at high voltagesand/or extreme temperatures. While screws securing one layer to anotherare suitable for lower voltage levels, such screws tends to result insignificantly changing the electrical characteristics of the chuck. Toovercome this limitation, all or a portion of, the layers of the chuckmay be partially or fully maintained together using a vacuum.

Referring to FIG. 9, the breakdown voltage between a pair of parallelplates, such as between a pair of conductive surfaces on either side ofa vacuum path, has a non-linear characteristic. The voltage necessary toarc across the gap decreases up to a point as the pressure is reduced.Then the voltage necessary to arc across a gap increases, graduallyexceeding its original value. In addition, decreasing the gap withnormal pressure results in the same behavior in the voltage needed tocause the arc. Accordingly, it is not desirable to include long vacuumlines, nor is it desirable to include vacuum lines that interconnect apair of conductive plates on either side of an insulating layer, nor isit desirable to include vacuum lines that otherwise pass through aregion of different voltage potentials.

Referring to FIG. 10, a vacuum line 550 may be attached to the upperchuck assembly element 502 which defines one or more paths 552 therein.The paths 552 preferably provide vacuum to the upper surface 504 so thatthe device under test 506 may be secured in place, while also providingvacuum to the lower surface 554 to maintain the upper chuck assemblyelement 502 to the upper chuck insulating layer 510. Accordingly, thedevice under test 506, the upper chuck assembly element 502, and theupper chuck insulating layer 510 are maintained in a fixed relationshipwith respect to one another while testing the device under test. Thevacuum provided to the upper surface 504 and the lower surface 554 maybe interconnected or may be isolated from one another. In addition, thevacuum provided to the upper surface may be selectively provided to oneor more zones depending on the size of the device under test 506.

Referring to FIG. 11, a vacuum line 560 may be attached to the upperchuck insulating layer 510 which defines one or more paths 562 therein.The paths 562 preferably provide vacuum to the middle chuck assemblyelement 508 to maintain the upper chuck insulating layer 510 to themiddle chuck assembly element 508. Accordingly, the device under test506, the upper chuck assembly element 502, the upper chuck insulatinglayer 510, and the middle chuck assembly element 508 are maintained in afixed relationship with respect to one another while testing the deviceunder test. In some cases, the middle chuck assembly element 508 mayinclude perforations therein. In this case, the vacuum is also providedto the lower chuck insulating layer 520, thereby maintaining it in afixed relationship with respect to the other layers.

Referring to FIG. 12, a vacuum line 570 may be provided to the lowerchuck insulating layer 520 which defines one or more paths 572 therein.The paths 572 preferably provide vacuum to the middle chuck assemblyelement 508 to maintain the lower chuck insulating layer 520 to themiddle chuck assembly element 508, if desired. The paths 572 preferablyprovide vacuum to the lower chuck assembly layer 522 to maintain thelower chuck insulating layer 520 to the lower chuck assembly layer 522,if desired. In some cases, the lower chuck assembly layer 522 mayinclude perforations therein. In this case, the vacuum is provided tothe thermal chuck 540, thereby maintaining it in a fixed relationshipwith respect to the other layers. Depending on the configuration, thelower chuck assembly layer 522 may be omitted.

A vacuum line 580 may be provided to the thermal chuck 540 which definesone or more paths 582 therein. The paths 582 preferably provide vacuumto the lower chuck assembly layer 522 to maintain the lower chuckassembly layer 522 to the thermal chuck 540. The paths 582 aremaintained within the thermal chuck 540 so the paths 582 are in a zonehaving the same potential, normally a shield and/or ground potential. Inthe case that the lower chuck assembly layer 522 includes perforationstherein, the thermal chuck 540 is maintained in a fixed relationshipwith the lower chuck assembly layer 522 and the lower chuck insulatinglayer 520.

In general, the upper chuck assembly element includes vacuum pathstherein. The upper chuck insulating layer 510 and/or the lower chuckinsulating layer 520 and/or the thermal chuck 540 may include vacuumpaths therein to suitable layers in order to maintain the integrity ofthe chuck 500. In some configurations, a ring 600 is provided thatencircles the chuck 500. The ring 600 is preferably interconnected to aguard potential which is the same potential to which the vacuum line 560and paths 562 extent to, namely, the guard potential of the middle chuckassembly element 508.

Referring to FIG. 13, the preferred arrangement of vacuum paths is suchthat there are no paths that extend from one side of an insulating layerto the other side of the insulating layer, thereby reducing thelikelihood of an arc between the two sides of the insulating layer. Withperforations within the middle chuck assembly element 508 and/or thelower chuck assembly layer 522, additional layers may be securedtogether while reducing the number of vacuum paths. In many cases, thesurfaces of the layers may include paths defined therein to more evenlydistribute the vacuum between the surfaces.

With a suitable chuck 500 configuration in a coaxial mode, 10,000 voltsmay be applied at 300 degrees Celsius without a substantial corona orarc discharge. The coaxial mode includes a signal applied to the upperchuck assembly element while the middle chuck assembly element and thelower chuck assembly element/thermal chuck are at a ground potential. Inthis configuration, the current leakage is preferably less than10,000/1010

With a suitable chuck 500 configuration in a triaxial mode, 3,000 voltsmay be applied at 300 degrees Celsius without a substantial corona orarc discharge. The triaxial mode include the upper chuck assemblyelement connected to a signal, the middle chuck assembly elementconnected to a guard potential, and the lower chuck assemblyelement/thermal chuck to a ground potential. In this (or other)configuration, the leakage current is preferably less than 10 pica amps,and more preferably less than 3 pica amps.

While the configuration of the boron nitride upper chuck insulatingelement 510 with an extension 516 reduces the high voltage breakdown, atsufficiently high temperatures and/or sufficiently high voltages asignificant offset current results in the measurements. After analysisof the structure of the chuck together with the materials, it wasdetermined that at least a partial source of the offset current is aresult of stresses induced in the upper chuck insulating element 510.The center region of the upper chuck insulating element 510 has atemperature consistent with that applied by the thermal chuck 540 whilethe extension 516 has a temperature somewhat lower than the temperatureof the central region of the upper chuck insulating element 510,resulting in a differential temperature. In addition to thisdifferential temperature, the central region of the upper chuckinsulating element 510 is maintained in position using a vacuum so thatstresses are induced between the central region of the upper chuckinsulating element 510 and the extension 516 region of the upper chuckinsulating element 510. These induced stresses in the upper chuckinsulating element 510 result in an induced voltage and current. Theinduced current tends to be on the order of 100 pica amps which isproblematic for low current measurements.

Referring to FIG. 14, a modified upper chuck insulating element 900includes one or more recesses 910 (otherwise the same as the upper chuckinsulating element 510) defined therein proximate the exterior edge ofthe upper chuck assembly element 502 and/or exterior edge of the middlechuck assembly element 508. The recesses 910 may extend around amajority of, substantially all of, and/or all of the circumference ofthe upper chuck insulating element 900. One or more recesses 910 may beincluded on the upper side of the upper chuck insulating element, and/orthe lower side of the upper chuck insulating element 900, and/or boththe upper and lower sides of the upper chuck insulating element 900. Asstresses are induced in the modified upper chuck insulating element 900the recesses 910 tend to contract thus absorbing the stresses that wouldotherwise be induced in the upper chuck assembly element 900.Preferably, the recesses 910 extend at least % of the thickness of theupper chuck insulating element 900, and more preferably of the thicknessof the upper chuck insulating element 900. In this manner, the offsetcurrent is significantly reduced.

Referring to FIG. 15, a modified upper chuck assembly element 920 may besized such that it has a smaller diameter than both the upper chuckinsulating element 900 and the middle chuck assembly element 508. Inthis manner, the temperature provided by the thermal chuck 540 may bemore evenly distributed over the upper surface 504 of the upper chuckassembly element 920 to the device under test 506.

Referring to FIG. 16, a modified upper insulating element 940 mayinclude a first thickness across a majority of its diameter and athinner thickness proximate the extension 516 portion of the upperinsulating element 940. The thinner thickness reduces the stresses thatare induced in the modified upper insulating element 940.

When automatic wafer loading/unloading equipment is used, wafer liftpins are incorporated into the chuck. Referring to FIG. 17, a liftmechanism 1750 pushes lift pin 1707 upward through a lift pin hole 1740in the wafer-supporting surface of the chuck top-force layer 1701. Asshown in FIG. 17, a high voltage triaxial thermal chuck 1700 preferablyincludes an insulated and guarded lift pin 1707 in an insulated sleeve1706 with the flange 1720 of the sleeve extending under theforce-to-guard isolation layer 1702 to maintain the flashover (orarcing) and creepage distance required for high voltage applications.The flange 1720 preferably extends under the force-to-guard isolationlayer 1702 so as to increase creepage distance between the conductivechuck top-force layer 1701 and the guard foil 1703. The sleeve 1706 mayinclude internal threads 1709 or other surface area increasing features(e.g. sinusoidal or square wave shaped physical structure) to provideadditional creepage distance between conductors. The sleeve 1706 ispreferably positioned and sized so that it cannot make contact with theguard-to-shield isolation layer 1704 or the chuck shield/ground layer1705. If the force-to-guard isolation layer 1702 comprises air or ahighly resistive film as for application in a non-thermal chuckconfiguration, then the flange 1720 of the sleeve 1706 preferablyextends far enough to maintain the flashover and creepage distancesrequired for the voltage in the area of the lift pin hole 1740. The liftpin 1707 is preferably made from an insulation material, and the liftpin 1707 may or may not have a guard layer, shown in FIG. 17 as a wire1730 from the guard foil 1703 and connected to a lower portion 1708 ofthe lift pin 1707.

Referring to FIG. 18, a high voltage non-thermal coaxial chuck 1800preferably includes an insulated lift pin 1804 in an insulated sleeve1802 with the flange 1820 of the sleeve extending under the chucktop-force layer 1801 to maintain the flashover (or arcing) and creepagedistance required for high voltage applications. The flange 1820preferably extends under the chuck top-force layer 1801 so as toincrease creepage distance between the conductive chuck top-force layer1801 and the chuck shield/ground layer 1803. The sleeve 1802 may includeinternal threads 1806 or other surface area increasing features (e.g.sinusoidal or square wave shaped physical structure) to provideadditional creepage distance between conductors. The flange 1820, asshown in FIG. 18, incorporates a square wave shaped structure to provideadditional creepage distance between the chuck top-force layer 1801 andthe chuck shield/ground layer 1803. The flange 1820 shape also providesgreater vertical separation to reduce high voltage arcing betweenconductors. If the force-to-guard isolation layer (not shown) comprisesair or a highly resistive film as for application in a non-thermal chuckconfiguration, then the flange 1820 of the sleeve 1802 preferablyextends far enough to maintain the flashover and creepage distancesrequired for the voltage in the area of the lift pin hole 1840.

For typical triaxial measurements, the potential of the guard layer isheld by a source-measure unit (SMU) to within a small potential of theforce, typically within millivolts. A “quasi triaxial” measurement or“quasi guard” technique may be used for coaxial measurements whereby theguard potential is set (or held by a SMU) to a portion of the chuck toppotential. For example, if the chuck top is at 10 kV, the guard may beset at 5 kV to effectively lower the breakdown potential by half. Thatis, if the chuck top is at 10 kV and the guard is set (or held) at 5 kV,the breakdown potential is effectively reduced from 10 kV to 5 kV,allowing chucks and probes to be more cost effectively manufactured andlowering the current leakage, leakage settling time, and noise forcoaxial measurements.

In high voltage operation, the wafer-supporting surface of the chuck maybe biased up to 10 kV and preferably does not breakdown electrically orexhibit electrical discharge to surrounding guard or shield structures.High voltage isolation between conductors is normally achieved byincreasing the distance between the two conductors, both in terms of theair gap between the conductors and the creepage distance across thesurface of any insulator that separates the conductors. Typically, thickisolators are used and are designed with a sinusoidal or square waveshaped edges to increase the surface creepage distance. At standardatmosphere and pressure, the creepage distance is typically the morerestrictive design criteria for conductors separated by a physicalisolator. For conductors remotely mechanically supported or wellisolated, the air gap may be the limiting factor.

As the temperature and voltage increase, sharp edges of surfaces thatare at high voltage potential have high intensity electric fields andcan emit electrons in a corona discharge. Such discharge can disruptleakage measurements and lead to high voltage breakdown.

Air spacing may be used for isolating various surrounding guard orshield structures. However, the air gap allowable is often limited bythe physical constraints and size of the probe station and test chamber.Use of thick isolation material to prevent arcing may be used. However,if the isolator is not a low dielectric absorption material, there canbe surface charge retention on the isolator that may become a cause oflow noise measurement errors. Consequently, for low noise measurements,any surface facing the measurement conductor should not be able to holda surface charge.

To ensure conductors in close proximity to the measurement surfaces ofthe chuck are sufficiently isolated, a thin nonconductive, lowdielectric absorption coating is preferably applied on one or more ofthe following surfaces of the thermal chuck measurement system, as shownin FIGS. 19a -19 c: the test chamber walls 1905, the guard ring 1901,the guard 1908, and preferably all of the guarding and shieldingsurfaces in close proximity to and not in direct contact with the chuckforce/sense measurement surface 1902. Test chamber upper surfaces 1906and upper structure 1907 are preferably similarly coated. Thenonconductive coating, in addition to resisting arcing, also effectivelycontains the highest intensity electric fields, quenching the coronadischarge effect.

In a preferred embodiment, the guard ring 1901, as shown in FIGS. 19a-19 c, is coated with Hyphlon MFA, or PTFE, or equivalent highisolation, low dielectric absorption coating in the manufactures'recommended thickness for high voltage, in this case 0.005″ or more. Thelow dielectric absorption and thin material allows surface charges todissipate quickly, while the high isolation properties protect againsthigh voltage arcing.

Other preferred embodiments include coating various combinations ofchamber wall 1905 and upper surfaces 1906, guard plate 1908 and guardring 1901, and/or other structures near the chuck such as the Aux chucks1903, alignment cameras (not shown), or other structures (e.g. screwhead 1904).

The chuck force/sense measurement surface 1902 is preferably connected,as shown in FIG. 19 a, to force and sense cables in a Kelvin connection1950 via an insulating or non-conductive block. Preferably, the Kelvinconnection 1950 provides very low resistance, below a milliohm, in orderto make low noise and or high current measurements, and maintain itsoperating characteristics in a highly stable manner over the fulloperating temperature range of the chuck (for example, from −65 Celsiusto 300 Celsius) and over the operational life time of the high voltagechuck, and rejecting electrical breakdown at high voltage to surroundingstructures.

Some designs use (for example, stainless steel or steel) hardware toconnect the measurement wires/leads either directly to the chuck top orwith a longer screw through an isolator block of a high temperatureengineering isolation material (such as, for example, plastic orpolyimide) or a ceramic material to the chuck. The difference betweenthermal expansion characteristics of the stainless steel screw and thealuminum chuck top, and/or the isolator block and stainless screw maycause the connection to loosen over time or with thermal cycling. Hightemperature engineering isolation materials typically have a coefficientof thermal expansion (CTE) greater than the screw and may yield somewhatat high temperatures causing a loss of clamping force and, thus, a lossin electrical contact in the bolted joint when temperatures fall backdown. Ceramic isolators typically have a CTE much lower than the screwand may lose clamping force and therefore contact at high temperatures.

The resistance of a screw may be calculated by multiplying the materialresistivity with the diameter and again the length of the screw.Stainless steel screws and their connection capabilities tend not toprovide low enough resistance for advancing technologies insemiconductor wafer testing instruments and equipment. The disadvantageof such designs is compounded by the physical limits of screw diametersthat can be used on the chuck top edge. Additionally, it is generallyundesirable to increase the thickness of the chuck top layer toaccommodate larger screw diameters due to the impact such increase hason the thermal mass and, thus, the thermal response of the chuck system.

Referring to FIGS. 20a -20 e, a first group of preferred embodiments forthe Kelvin connection 1950 include an insulating block 2080 or otherelectrically non-conductive cover attached to the chuck top 1902 inorder to protect the measurement connections 2010 from arcing tosurrounding structures. The measurement connections 2010 preferably passthrough the insulating block 2080 without depending on the insulatingblock for clamping force and therefore electrical contact. Allcomponents of the electrical connection 2010 through the insulationblock 2080 preferably have matching GTE's and, most preferably, are ofthe same material, for example, brass.

A first embodiment of this first group is shown in FIGS. 20a-20e andpreferably includes a block 2080 of electrically insulating materialattached to the chuck top 1902 with fasteners 2060 that are not theelectrical connection. The electrical connection is made by a screw 2070that extends thru a hollow standoff that extends beyond the insulatingblock 2080 and tightens onto the standoff to hold a continuous clampingforce and therefore electrical contact to the chuck 1902. The screw,sleeve and washer 2040 materials are matched (for example, all made ofbrass) to better match the GTE of the aluminum chuck top 1902, and theadditional conductor area added by the sleeve 2030 reduces theresistance of the connection by two to four times over the materialchange (from conventional designs using stainless steel) alone for agiven screw size.

A second embodiment of this first group preferably includes a block ofelectrically insulating material attached to the chuck top 1902 withfasteners 2060 that are not the electrical connection. The electricalconnection is made by an at least partially threaded stud 2070 thatextends thru a hollow standoff and has a nut that tightens onto thestandoff to hold a continuous clamping force and therefore electricalcontact to the chuck 1902. The threaded stud, sleeve, nut, and washermaterials are all preferably matched (and preferably are made of brass)to better match the GTE of the aluminum chuck top 1902, and theadditional conductor area added by the sleeve 2030 reduces theresistance of the connection by two to four times over the materialchange alone for a given screw size. Additional nuts may be used to holdthe wire lugs 2050 for attachment to measurement cables.

A third embodiment of this first group preferably includes a block ofelectrically insulating material attached to the chuck top 1902 withfasteners 2060 that are not the electrical connection. The electricalconnection is made by an at least partially threaded stud 2070 that hasa nut that tightens onto the chuck 1902 to hold a continuous clampingforce and therefore electrical contact to the chuck. An additional pairof nuts may be used to hold the wire lugs 2050 for attachment tomeasurement cables. The screw, sleeve, nuts, and washer materials arepreferably matched and are preferably brass, to better match the CTE ofthe aluminum chuck top 1902.

A fourth embodiment of this first group preferably includes a block ofelectrically insulating material attached to the chuck top 1902 withfasteners 2060 that are not the electrical connection. The electricalconnection is made by a threaded standoff that screws into the chuck top1902 or onto a stud in the chuck top 1902 and additionally a screw isused to hold the wire lugs 2050 for attachment to measurement cables tohold continuous clamping force and therefore electrical contact to thechuck. The screw, standoff, and washer materials are matched andpreferably made of brass, to better match the CTE of the aluminum chucktop 1902, and the additional conductor area added by the threadedstandoff reduces the resistance of the connection by two to four timesover the material change alone for a given screw size.

A fifth embodiment of this first group preferably includes a block ofelectrically insulating material attached to the chuck top 1902 withfasteners 2060 that are not the electrical connection. The electricalconnection is made from a block of conductive material attached to thechuck top 1902 and enclosed within the insulating block on at least 3sides. A screw is used to hold the wire lugs 2050 for attachment tomeasurement cables to hold continuous clamping force and thereforeelectrical contact to the chuck 1902. The screw, conductive block, andwasher materials are matched and in this case brass, to better match theCTE of the Aluminum chuck top, the additional conductor area added bythe connection block reduces the resistance of the connection by atleast two to four times over the material change alone for a given screwsize.

Still referring to FIGS. 20a -20 e, a second group of preferredembodiments for the Kelvin connection 1950 include an insulating sleeve2030 or other electrically non-conductive cover attached over themeasurement connections to prevent arcing to surrounding structures,without attaching the sleeve 2030 to the chuck top 1902. The measurementconnections 2010 preferably pass through the insulating sleeve 2030without depending on the insulating sleeve for clamping force andtherefore electrical contact. All components of the electricalconnection through the insulation sleeve preferably have matching CTE'sand, most preferably, are of the same material, for example, brass.

A first embodiment of this second group preferably includes aninsulating sleeve 2030 or other electrically non-conductive coverattached over the measurement connections to prevent from arcing tosurrounding structures without attaching the sleeve 2030 to, the chucktop 1902. The electrical connection is preferably made by a screw 2070that extends thru a hollow standoff that extends beyond the insulatingsleeve 2030 and tightens onto the standoff to hold a continuous clampingforce and therefore electrical contact to the chuck 1902. The screw,hollow standoff, and washer materials are preferably matched and morepreferably made of brass, to better match the CTE of the aluminum chucktop 1902, and the additional conductor area added by the sleeve reducesthe resistance of the connection by two to four times over the materialchange alone for a given screw size.

A second embodiment of this second group preferably includes aninsulating sleeve 2030 or other electrically non-conductive coverattached over the measurement connections to prevent from arcing tosurrounding structures without attaching the sleeve 2030 to the chucktop 1902. The electrical connection is preferably made by an at leastpartially threaded stud 2070 that extends thru a hollow standoff and hasa nut that tightens onto the standoff to hold a continuous clampingforce and therefore electrical contact to the chuck. The screw,partially threaded stud, hollow standoff, nut and washer materials arepreferably matched and preferably comprise brass, to better match theCTE of the aluminum chuck top 1902, and the additional conductor areaadded by the sleeve 2030 reduces the resistance of the connection by twoto four times over the material change alone for a given screw size.Additional nuts can be used to hold the wire lugs 2050 for attachment tomeasurement cables.

A third embodiment of this second group preferably includes aninsulating sleeve 2030 or other electrically non-conductive coverattached over the measurement connections to prevent them from arcing tosurrounding structures without attaching the sleeve 2030 to the chucktop 1902. The electrical connection is preferably made by an at leastpartially threaded stud 2070 that has a nut that tightens onto the chuck1902 to hold a continuous clamping force and therefore electricalcontact to the chuck 1902. An additional pair of nuts may be used tohold the wire lugs 2050 for attachment to measurement cables. The screw,partially threaded stud, nuts and washer materials are preferablymatched and preferably made of brass, to better match the CTE of thealuminum chuck top 1902.

A fourth embodiment of this second group preferably includes aninsulating sleeve 2030 or other electrically non-conductive coverattached over the measurement connections to prevent them from arcing tosurrounding structures without attaching the sleeve 2030 to the chucktop 1902. The electrical connection is preferably made by a threadedstandoff that screws into the chuck top 1902 or onto a stud in the chucktop 1902 and additionally a screw is used to hold the wire lugs 2050 forattachment to measurement cables to hold continuous clamping force andtherefore electrical contact to the chuck 1902. The screw, standoff, andwasher materials are preferably matched and preferably made of brass, tobetter match the CTE of the aluminum chuck top 1902, and the additionalconductor area added by the threaded standoff reduces the resistance ofthe connection by two to four times over the material change alone for agiven screw size.

A fifth embodiment of this second group preferably includes aninsulating sleeve 2030 or other electrically non-conductive coverattached over the measurement connections to prevent arcing tosurrounding structures without attaching the sleeve 2030 to the chucktop 1902. The electrical connection is made from a block of conductivematerial attached to the chuck top 1902 and enclosed within theinsulating sleeve on at least 3 sides. A screw is used to hold the wirelugs 2050 for attachment to measurement cables to hold continuousclamping force and therefore electrical contact to the chuck 1902. Thescrew, conductive block, and washer materials are preferably matched andpreferably made of brass, to better match the CTE of the aluminum chucktop 1902, and the additional conductor area added by the connectionblock reduces the resistance of the connection by at least two to fourtimes over the material change alone for a given screw size.

The terms and expressions which have been employed in the foregoingspecification are used therein as terms of description and not oflimitation, and there is no intention, in the use of such terms andexpressions, of excluding equivalents of the features shown anddescribed or portions thereof, it being recognized that the scope of theinvention is defined and limited only by the claims which follow.

The invention claimed is:
 1. A chuck comprising: (a) an upper conductivelayer having a lower surface and an upper surface suitable to contactand support a device under test, wherein said upper surface of saidupper conductive layer includes thereon a vacuum distribution forholding said device under test, wherein the vacuum distribution isconfigured to selectively and independently apply a vacuum to multipledistinct zones of the upper surface of the upper conductive layer; (b)an upper insulating layer having an upper surface at least in partialface-to-face contact with said lower surface of said upper conductivelayer, and a lower surface, wherein the upper insulating layer ishorizontally distinct from the upper conductive layer; and (c) a middleconductive layer having an upper surface at least in partialface-to-face contact with said lower surface of said upper insulatinglayer, and a lower surface, wherein the middle conductive layer ishorizontally distinct from the upper insulating layer.
 2. The chuck ofclaim 1, wherein said vacuum distribution comprises a plurality ofvacuum holes positioned across said upper surface of said upperconductive layer and close enough to one another to have overlappingareas of full vacuum effect so as to provide uninterrupted vacuum acrosszones of said upper surface of said upper conductive layer.
 3. The chuckof claim 2, wherein said plurality of vacuum holes is uniformly spacedon said upper surface of said upper conductive layer.
 4. The chuck ofclaim 2, wherein each of said plurality of vacuum holes is less than0.38 inches away from a closest other vacuum hole of the plurality ofvacuum holes.
 5. The chuck of claim 2, wherein the plurality of vacuumholes is positioned to provide uninterrupted vacuum across said uppersurface of said upper conductive layer.
 6. The chuck of claim 2, whereinthe chuck further includes a plurality of cross-drilled inner channels,wherein each of the plurality of cross-drilled inner channels isconfigured to conduct a respective vacuum to a respective subset of theplurality of vacuum holes.
 7. The chuck of claim 2, wherein said lowersurface of said upper conductive layer includes a plurality of channelsand a plurality of machined holes, wherein each of the plurality ofmachined holes interconnects a selected one of the plurality of vacuumholes with a corresponding one of the plurality of channels.
 8. Thechuck of claim 2, wherein the plurality of vacuum holes includes aplurality of machined vacuum holes.
 9. The chuck of claim 1, wherein thevacuum distribution includes a plurality of vacuum lines defined by theupper conductive layer, wherein the vacuum distribution further includesa plurality of vacuum holes positioned across said upper surface of saidupper conductive layer, and further wherein each of the plurality ofvacuum lines is configured to apply a corresponding vacuum to acorresponding subset of the plurality of vacuum holes.
 10. The chuck ofclaim 9, wherein each corresponding subset of the plurality of vacuumholes is distinct from each other corresponding subset of the pluralityof vacuum holes.
 11. The chuck of claim 9, wherein the multiple zonesare arranged in concentric circles on the upper surface of the upperconductive layer.
 12. The chuck of claim 9, wherein the plurality ofvacuum holes includes a plurality of machined vacuum holes.
 13. Thechuck of claim 1, wherein said vacuum distribution comprises a poroussintered metal layer having at least as small as one micron sized poresthrough which a vacuum is drawn.
 14. The chuck of claim 1, wherein saidvacuum distribution comprises micro-grooves at least as small as 50microns wide and 15 microns deep, said micro-grooves formed upon saidupper surface of said upper conductive layer with close enough spacingso as to provide a substantially continuous vacuum field across zones ofsaid upper surface of said upper conductive layer.
 15. The chuck ofclaim 1, wherein no vacuum path is provided that extends from said uppersurface of said upper insulating layer to said lower surface of saidupper insulating layer.
 16. The chuck of claim 1, wherein said upperinsulating layer includes a recess defined therein, wherein said recessis proximate an exterior peripheral surface of at least one of saidupper conductive layer and said middle conductive layer and extendsaround a majority of a circumference of said upper insulating layer. 17.The chuck of claim 1, further comprising an insulated lift pin slidablyextendable from a retracted position within a lift pin hole formedwithin said upper surface of said upper conductive layer, and aninsulated lift pin sleeve further interposed between said insulated liftpin and said upper conductive layer, wherein said insulated lift pinsleeve includes a flange projecting radially outward from an axis ofsaid insulated lift pin, said flange adapted in size and shape toincrease at least one of an arc distance and a creepage distancecharacteristic of said chuck.
 18. The chuck of claim 1, wherein saidmiddle conductive layer comprises a guard potential held to apredetermined value, wherein said predetermined value is between asignal potential and a shield potential.
 19. The chuck of claim 1,wherein said upper insulating layer extends beyond the exteriorperipheral surface of said upper conductive layer and the exteriorperipheral surface of said middle conductive layer.
 20. The chuck ofclaim 1, wherein said upper insulating layer extends beyond the exteriorperipheral surface of said upper conductive layer and the exteriorperipheral surface of said middle conductive layer, and wherein saidupper conductive layer has a smaller diameter than said middleconductive layer.
 21. The chuck of claim 1, wherein the chuck is anassembly of separate parts, wherein the separate parts include the upperconductive layer and the upper insulating layer.
 22. A chuck comprising:(a) an upper conductive layer having a lower surface and an uppersurface suitable to contact and support a device under test, whereinsaid upper surface of said upper conductive layer includes thereon avacuum distribution for holding said device under test, wherein saidvacuum distribution comprises a plurality of vacuum holes positionedacross said upper surface of said upper conductive layer and closeenough to one another to have overlapping areas of full vacuum effect soas to provide uninterrupted vacuum across zones of said upper surface ofsaid upper conductive layer, and further wherein said plurality ofvacuum holes is uniformly spaced on said upper surface of said upperconductive layer; (b) an upper insulating layer having an upper surfaceat least in partial face-to-face contact with said lower surface of saidupper conductive layer, and a lower surface, wherein the upperinsulating layer is horizontally distinct from the upper conductivelayer; and (c) a middle conductive layer having an upper surface atleast in partial face-to-face contact with said lower surface of saidupper insulating layer, and a lower surface, wherein the middleconductive layer is horizontally distinct from the upper insulatinglayer.
 23. The chuck of claim 22, wherein each of said plurality ofvacuum holes is less than 0.38 inches away from a closest other vacuumhole of the plurality of vacuum holes.
 24. The chuck of claim 22,wherein the plurality of vacuum holes is positioned to provideuninterrupted vacuum across said upper surface of said upper conductivelayer.
 25. The chuck of claim 22, wherein the chuck further includes aplurality of cross-drilled inner channels, wherein each of the pluralityof cross-drilled inner channels is configured to conduct a respectivevacuum to a respective subset of the plurality of vacuum holes.
 26. Thechuck of claim 22, wherein said lower surface of said upper conductivelayer includes a plurality of channels and a plurality of machinedholes, wherein each of the plurality of machined holes interconnects aselected one of the plurality of vacuum holes with a corresponding oneof the plurality of channels.
 27. The chuck of claim 22, wherein theplurality of vacuum holes includes a plurality of machined vacuum holes.28. The chuck of claim 22, wherein said upper insulating layer includesa recess defined therein, wherein said recess is proximate an exteriorperipheral surface of at least one of said upper conductive layer andsaid middle conductive layer and extends around a majority of acircumference of said upper insulating layer.
 29. A chuck comprising:(a) an upper conductive layer having a lower surface and an uppersurface suitable to contact and support a device under test, whereinsaid upper surface of said upper conductive layer includes thereon avacuum distribution for holding said device under test; (b) an upperinsulating layer having an upper surface at least in partialface-to-face contact with said lower surface of said upper conductivelayer, and a lower surface, wherein the upper insulating layer ishorizontally distinct from the upper conductive layer; and (c) a middleconductive layer having an upper surface at least in partialface-to-face contact with said lower surface of said upper insulatinglayer, and a lower surface, wherein the middle conductive layer ishorizontally distinct from the upper insulating layer; wherein saidupper insulating layer includes a recess defined therein, wherein saidrecess is proximate an exterior peripheral surface of at least one ofsaid upper conductive layer and said middle conductive layer and extendsaround a majority of a circumference of said upper insulating layer.